Metal surface coating has played an important role in extending the life cycle of structural materials commonly used in large rugged equipment for use on land, air, and sea. Aluminum and its many versatile alloys are routinely used as surface coatings for the corrosion protection of many metals, offering both barrier and sacrificial protection. In addition, aluminum and its alloys are being considered as favorable alternatives for cadmium coatings on the protective shells of electrical connectors in military ground systems in view of the known toxic and carcinogenic nature of cadmium and hexavalent chromium materials.
Currently, there are various methods for aluminum deposition, such as hot dipping, thermal spraying, sputter deposition, vapor deposition, and electrodeposition. However, a particularly attractive method for depositing aluminum and its alloys is isothermal electrodeposition, either by tank or brush plating. Electrodeposition is an attractive technique because it generally leads to thin, economical coatings that are usually adherent and do not affect the structural and mechanical properties of the substrate. Moreover, the thickness and quality of the deposits can be controlled by adjustment of the deposition rate by tuning such experimental parameters as overvoltage, current density, electrolyte composition, and temperature.
Unfortunately, neither aluminum nor its alloys can be electrodeposited from aqueous solutions because hydrogen is evolved before aluminum can be plated. Thus, it is necessary to employ non-aqueous solvents (both molecular and ionic) for this purpose. On a commercial basis, aluminum is plated by using the well known SIGAL® process. Although known to be very effective, the SIGAL® process requires a plating bath composed of alkyl aluminum fluorides dissolved in toluene. Not surprisingly, the technique raises a number of environmental and safety objections because the alkyl aluminum compounds are pyrophoric and toxic, and the toluene solvent is flammable and can lead to volatile organic compound (VOC) emissions. The inefficiency of aqueous electroplating also makes it a major energy consumer (for example, for electrolytic hard chrome plating, only 10-20% of the power supplied is used for actual deposition; the remaining power is consumed through hydrogen generation and other losses).
More recently, aluminum-containing ionic liquids (i.e., aluminum-containing molten salts) have gained increasing prominence as substantially improved electrolytes for the deposition of aluminum. The ionic liquids possess an advantageous combination of physical properties, including non-flammability, negligible vapor pressure, high ionic conductivity, and high thermal, chemical, and electrochemical stability. Therefore, they are amenable for the electroplating of reactive elements, which is impossible using aqueous or other organic solvents. Thus far, the ionic liquids used for the electrodeposition of aluminum has focused on chloroaluminate anions, which are typically obtained by mixing anhydrous AlCl3 with an organic chloride salt, such as 1-ethyl-3-methyl imidazolium chloride (EMImCl), 1-(1-butyl)pyridinium chloride (N-BPCl), or other related salt. However, because of the hygroscopic nature of AlCl3 and the resulting chloroaluminate, the electroplating generally must be performed in an inert gas atmosphere, which significantly increases cost and complexity of the process. In addition, the anionic nature of the electroactive species in the ionic liquid (e.g., Al2Cl7−) presents a significant hindrance in the ability of the electroactive species to accept electrons for aluminum deposition, which further decreases the efficiency of the process.